1. Field of the Invention
This invention relates to an integrated cell having excellent portability and maintenance property which uses integrated lithium secondary cells to obtain a desired voltage and a desired current capacity, and to a wound type cell produced by winding thin sheet-like electrode plates in a spiral form onto a square cell having a high energy density such as a lithium secondary cell, a nickel hydrogen cell, etc., through separators. More particularly, the present invention relates to a chemical cell having a safety mechanism for restricting an abnormal reaction at leads extended from the electrode plates.
2. Description of the Related Art
An output voltage of a single cell is primarily determined by materials constituting the cell. To obtain a desired voltage value, therefore, a series circuit must be formed using a plurality of such cells. To increase a current capacity (Ah), on the other hand, a parallel circuit must be formed. A desired voltage value and a desired capacity can be obtained by integrating a plurality of unit cells into a so-called "integrated cell" in this way.
A mean output voltage of a single cell is about 3 V in the case of a lithium cell. To obtain a cell having a voltage of 240 V and a current capacity of 100 Ah, for example, eighty unit cells of 3 V and 100 Ah are connected in serries, or two serries circuits each comprising eighty unit cells of 3 V and 50 Ah are connected in parallel. Therefore, a large number of unit cells are arranged inside a case using a metal structural material having high strength, and the terminals of each unit cell are connected by bolts, nuts, leads, etc., so as to form a series-parallel circuit of the unit cells.
However, the integrated cell according to the prior art involves the following problems.
In the prior art cell devices accommodating all the unit cells in the single case, the weight and the capacity of the integrated cell increase when the voltage and the current capacity increase, so that transportation and movement becomes difficult. On the other hand, the secondary cell must be regularly charged, and portability of the cell is desired so that the operation can replace the cells during a charging operation.
When a large number of cells are collectively wired, the wiring becomes complicated and the assembly work, as well as maintenance work, becomes more difficult.
In the integrated secondary cell, inspection and maintenance is necessary so as to check and exchange defective cells and used cells. Therefore, this inspection and maintenance must be facilitated.
In view of the problems with the prior art cells describe above, the present invention aims at providing a lithium secondary integrated cell which is portable and easy to maintain.
One of the known cells having a high energy density is a cylindrical cell which is produced by winding four kinds of belt-like unit laminate sheets, that is, a positive electrode material sheet containing an electrolyte, a separator sheet, a negative electrode material sheet and a separator sheet, into a cylindrical shape, and storing and holding the resulting cylindrical cell elements in a cylindrical cell case.
Lithium metal, which has the basest potential and the greatest energy density per unit weight and unit volume has drawn attention as a negative electrode material for a secondary cell system directed to accomplish a higher energy density. A lithium-containing metal oxide such as LiMn.sub.2 O.sub.4, etc., providing a high energy density, particularly a spinel type compound, has drawn attention as the positive electrode material for the negative pole described above. Such a lithium secondary cell is reported in Japanese Unexamined Patent publication (Kokai) No. 2-139860, for example.
The cylindrical cell as a single cell has a high energy density. However, when a plurality of these cylindrical cells are arranged in a cell case, unnecessary spaces are defined between the cells, and a space occupying ratio cannot be improved. Rectangular square secondary cells are more advantageous to improve the space occupying ratio when a plurality of cell are arranged. However, the square secondary cells, particularly the square secondary cells having a large current capacity, are not free from the following problems. For, since a rectangular unit cell element is formed by laminating a rectangular positive pole active material sheet containing an electrolyte, a separator sheet, a negative pole active material sheet and a separator sheet, adhesion between these sheet is not high, and as the number of times of lamination increases, the number of unnecessary spaces between the sheets increase, so that the volume of the unit cell element increases. Accordingly, the unit cell elements must be accommodated in a larger cell case, packing efficiency thus drops and a cell having a high volume energy density cannot be obtained.
The present invention is directed to solve these problems, and to provide a square secondary cell having less unnecessary space between the sheets of the positive pole active material sheet, the separator sheet, the negative pole active material sheet and the separator sheet, that constitute the unit cell element.
As portable electronic devices and appliances have become wide spread in recent years, demands for a greater capacity and a higher energy density of batteries have been increasing. The key point for accomplishing high energy density in the cells of batteries resides in a structure and a shape capable of securing a large electrode plate area in a limited volume, and a wound cell has gained a wide application as an example of such cells.
In the ordinary wound cell, an electrode roll formed by winding positive and negative electrode plates is fitted into a can formed by contraction molding and serving as the negative pole, and the positive and negative leads from the electrode roll are connected to the upper cover as the positive pole terminal and the can as the negative pole terminal, respectively.
The positive and negative leads of the electrode roll are formed by coupling lead plates 2092 to the end portion 2091 of the electrode plates 2090 as shown in FIG. 29.
A method of bonding an insulating tape to both surfaces of the lead plate 2092 lest the lead plate 2092 electrically short-circuits to unnecessary portions inside the cell when the lead plate 2092 is connected has been proposed in the past (Japanese Unexamined Patent Publication (Kokai) No. 4-329259).
The separator for insulation is sandwiched between the positive and negative plates of the electrode roll, but in order to make the insulation between the electrode plates more reliable, a method which expands the width of the separator to be sandwiched between both electrode plates in the vertical direction has been proposed (Japanese Unexamined Patent Publication (Kokai) No. 3-25865).
Another method has been proposed which inserts a hollow cylindrical pin into the center gap portion of the electrode roll so as to reinforce the electrode roll stored in the cell can, and to improve the working factor in the assembly of the electrode roll to the cell can (Japanese Unexamined Patent Publication (Kokai) No. 4-332481).
However, the wind-up type cell according to the prior art involves the following problems.
The first problems resides in that the working factor for connecting the lead of the electrode plate to the bottom of the cell can is low, the number of man-hour is great, and defects in quality such as a short-circuit are likely to occur.
In other words, if the lead (which is generally the negative pole) to be connected to the can bottom is short, the connection and fixing work of the lead to the can bottom is extremely difficult because there is not a sufficient space margin. On the other hand, when the lead is made longer, deflection of the lead occurs after the electrode roll is fitted, comes into contact with portions other than the can bottom portion, and is likely to cause a short-circuit.
The second problems lies in that the structure in which the lead plate is connected to the end portion 2091 of the electrode plate 2090 generates a voltage drop and exothermicity when the electrode plate is elongated, as shown in FIG. 29. In other words, according to the method which disposes the lead plate at only one of the ends, the distance becomes great between the other end portion 2093 and the lead plate 2092, so that the internal resistance value of the electrode plate inclusive of the lead plate becomes great as a whole, and exothermicity (Joule heat) and a drop in voltage take place.
On the other hand, when a method which disposes a large number of lead plates 2092 and bundles them into one lead as shown in FIG. 30 is employed to lower the resistance value of the electrode plate, the internal resistance value of the electrode plate can be lowered as a whole and exothermicity and voltage drop can be suppressed.
According to this method, however, the number of lead plates increases and hence, the number of components increases. Further, the number of man-hours for fitting the lead plates to the electrode plate, and the number of man-hours for bundling the lead plates increase. As a result, this method results in an increase in the production cost.
In view of the problems with the prior art as described above, the present invention provides a wound type cell which has a low internal electrode resistance, and has lead extension portions which can be assembled easily.
If foreign matter enters a chemical cell from outside or if trouble such as short-circuit occurs, there occurs the case where the chemical reaction abnormally proceeds and invites a remarkable temperature rise or the generation of enormous quantities of a gas. Therefore, how to secure the safety of the cell is of utmost importance.
In the case of a lithium metal cell which is highly regarded as a cell having a high energy density, for example, large quantities of gas and a temperature rise are generated by the abnormal reaction resulting from the mixture of the moisture and precipitation of dendrite, and how to secure safety of the cell is a critical problem in order to put the lithium metal cell into practical application.
Therefore, an ordinary enclosed type cell is equipped with a gas emission mechanism to prevent sudden jetting of the gas or explosion. Various proposals have also been made as to how to suppress the progress of the abnormal reaction.
To prevent the progress of an abnormal reaction in the cell, a method has been proposed, for example, which separates a current collector rod inside a cell from a seal port cover which is a positive pole terminal when the internal temperature abnormally rises, and thus cuts off the current path inside the cell (Japanese Unexamined Patent Publication (Kokai) No. 57-67289).
In the lithium metal cell, a method which absorbs the hydrogen gas generated by the mixture of moisture with a hydrogen-absorbing alloy and prevents jetting of the gas has been proposed (Japanese Unexamined Patent Publication (Kokai) No. 60-258870).
However, the safety mechanisms of the cell according to the prior art described still involve the following problems.
First of all, the method which merely discharges the gas provides the effect of preventing the danger such as explosion, but it does not suppress the cell reaction. Accordingly, the generation of the gas itself does not stop, and this method does not provide a fundamental safety measure.
Similarly, the method which disposes hydrogen-absorbing alloy inside the lithium metal cell provides the effect of preventing jetting of the hydrogen gas, but the cell reaction does not stop. Since gases other than the hydrogen gas such as carbon dioxide cannot be absorbed, this method is not sufficient as a safety measure.
The method which pulls off the current collector rod from the seal port cover at the time of the abnormal rise of the temperature can suppress the reaction by cutting off the current against the temperature rise due to the external short-circuit, etc., but does not provide any effect for the internal short-circuit of the cell. In other word, in the case of the internal short-circuit such as the short-circuit between the electrode plates, a different current path from the current path between the current collector rod and the seal port cover is formed. Accordingly, the current is not cut off even when the current collector rod is pulled off from the seal port cover.
In view of the problems with the prior art described above, the present invention is directed to provide a chemical cell equipped with a safety mechanism which can reliably suppress the abnormal reaction of the cell inclusive of factors such as an internal short-circuit.
To cope with these problems, the ordinary enclosed type cell is equipped with the internal gas emission mechanism, and prevents drastic jetting of the gas and explosion. Various proposals have been made to suppress the progress of the abnormal reaction.
On the other hand, the method which separates the current collector rod from the seal port cover in response to the abnormal rise of the temperature (Japanese Unexamined Patent Publication (Kokai) No. 57-67289) provides the effect of suppressing the reaction by cutting off the current, but is not yet free from the problem how to reliably keep the cut-off state of the current.
In other words, this is the method which causes the current collector rod to fall into a recess by its own weight and to be pulled off from the seal port cover, thereby cutting off the current. Accordingly, when the cell is turned upside-down, that is, when the seal port cover exists on the down side, the current cannot be cut off.
Even if the seal port cover exists on the up side and the current is once cut off, the current collector rod comes again into contact with the seal port cover and becomes conductive when the cell is turned over.
Furthermore, since this method does not provide a mechanism for emitting the internal gas, there remains the danger of gas jetting or explosion before the cell reaction completely stops.
A method which causes the separator used for the cell reaction portion to be clogged at the time of the temperature rise and thus suppresses the cell reaction at the time of abnormality has been employed generally (Japanese Unexamined Patent Publication (Kokai) No. 3-25865). However, when the separator gets clogged, the internal resistance of the cell becomes extremely great. Accordingly, in the case of an aggregate (integrated) cell formed by connecting in series a plurality of cells, there occurs the problem that the aggregate cell does not function as a whole due to abnormality of the cell described above.
In view of these problems with the prior art described above, the present invention is directed to provide a chemical cell equipped with a safety mechanism which can suppress the abnormal reaction of the cell and can prevent, in advance, an explosion of the cell, and so forth.